In some cultures, children who are born as a result of interspecific relations are rejected by the society of both races. For example, Bajoran / Cardassian hybrids were often rejected by both Bajorans and Cardassians. (DS9: "Indiscretion", "Return to Grace")

In the novel The Ashes of Eden, Kirk discovered the colony world of Chal, a planet briefly colonized by the Romulans and Klingons during an alliance, populated by genetically-engineered Klingon-Romulan hybrids.

It may be assumed that a Vulcan and Romulan would be able to breed due to their common ancestry. the production notes from Enterprise state that the creators were planning to reveal T'Pol as a Romulan/Vulcan hybrid if the series had continued. In this case, her baby with Trip would be 1/4 romulan, 1/4 vulcan and 1/2 human. Saavik was intended to be half-Romulan in Star Trek II: The Wrath of Khan, but a scene establishing this was cut.